Imagine a block sliding on a surface including friction. Gradually, it'll stop. If we touch both the surface and the block, they are hotter, their temperatures have increased. I've read that kinetic energy has turned into heat, which doesn't sound accurate to me. I handle the next heat definition that is very common: Heat is the transfer of energy due to a diference of temperatures between to two systems. Regarding only about the fact that heat is a flow of energy, so it doesn't appear correct to argue kinetic energy is now heat, since kinetic energy is something you have, but heat isn't. I read a very nice answer that holds this: kinetic energy is transfered as heat, and now is thermal energy. Thermal energy has to do with internal energy. But internal energy may depend on very variables, but thermal energy is only related with temperature, right? Yet I find it confusing. Anyway, so, the total energy can be distributed to increase the temperature of both objtects in many ways, right? Does the same thing happen with collitions?
On the other hand, would it be equivalent saying the work that friction performs is now thermal energy? Finally, since heat is transfer of energy, (any kind of energy), if two systems are in contact, one hotter than the other, transfer of energy takes place, what kind of energy is transfered?. That's it.